i isolated pET20b vector from XL1 by plasmid midi kit.Then i ran an agarose gel electrophoresis and observed the plasmid very concentrated , then I performed digestion reaction by HindIII. But i saw extra band on the gel after digestion . I ve also tried another enzymes (NcoI,BamHI) but again observing 2 bands after electrophoresis.... Do you have any idea about the reasons of my problem?
thanks

sorry too little info. maybe u can supply more when data. what is the size of the plasmid you extracted? what is the size of both the bands u visualize in the gel? have u inserted any gene in the plasmid yet?